1. Field of the Invention
The invention relates to an inspection method for a direct liquid fuel cell generator that generates electricity by supplying a liquid fuel and an oxidant, an inspection apparatus for a direct liquid fuel cell generator using the inspection method, and a direct liquid fuel cell generator comprising the inspection apparatus of the fuel cell generator.
2. Description of the Related Art
Fuel cells are generators that directly convert a chemical energy (free energy of combustion reactions) into a direct electrical energy, and are expected to have a higher conversion efficiency than thermal power generation. While power generation efficiency decreases when the scale of thermal power generation is small, power generation by the fuel cell is not decreased in small scale operation. Accordingly, the fuel cell is suitable for small scale power generation.
Among the fuel cells, developments of solid polymer electrolyte fuel cells have been accelerated in recent years as automobile power sources and domestic power sources. A gas containing hydrogen is introduced into an anode side, and oxygen gas or air is introduced into a cathode side in this solid polymer electrolyte fuel cell. An electromotive force is generated by the following reactions represented by the chemical formulae 1 and 2 at the anode and cathode sides, respectively.Anode: 2H2→4H++4e−  chemical formula 1Cathode: O2+4H++4e−→2H2O   chemical formula 2
The equations mean that electrons and protons are formed from hydrogen by means of a catalyst within the anode. The electrons are taken out of the cell through external circuits, and are used for power generation. The protons move in the solid electrolyte membrane and arrive at the cathode, and water is generated by a reaction between the electrons, oxygen, and the protons by a catalyst within the cathode. Electric power is generated by such cell reaction.
On the other hand, a direct methanol fuel cell has been noticed in recent years. FIG. 1 shows the structure of the direct methanol fuel cell. In the direct methanol fuel cell, a proton conductive electrolyte membrane (a perfluorocarbon sulfonic acid ion exchange membrane; Nafion made by DuPont Co. is preferably used) is sandwiched between an anode electrode and a cathode electrode. Each electrode comprises a substrate and a catalyst layer which comprises a catalyst and a proton conductive electrolyte. The catalyst is usually a precious metal or an alloy of the precious metal, which is used by being supported on carbon black. The catalyst is not supported on carbon black in some cases. A Pt—Ru alloy is preferably used as catalyst at the anode side, while Pt is preferably used as catalyst at the cathode side. Methanol and water are introduced into the anode side, and oxygen gas or air is introduced into the cathode side for operation. The reactions represented by the following chemical formulae 3 and 4 occur at the anode and cathode sides, respectively.Anode: CH3OH+H2O→CO2+6H++6e−  chemical formula 3Cathode: (3/2) O2+6H++6e−→3 H2O   chemical formula 4
These equations mean that electrons, protons and carbon dioxide are formed by the catalyst in the anode catalyst layer. Carbon dioxide generated is exhausted in the atmosphere. The electrons are taken out of the fuel cell through an external circuit, and are used for power generation. The protons move in a proton conductive electrolyte membrane, and arrive at the cathode. Water is formed in the cathode catalyst layer by a reaction of the electrons and oxygen and protons. The operating temperature of this direct methanol fuel cell is usually 50 to 120° C.
It was a drawback that a reformer should be provided in the fuel cell system and the entire system is forced to be large size, when a gas containing hydrogen is used as a fuel as in the solid polymer electrolyte fuel cell as described above, since the hydrogen gas is generally obtained by reforming methanol, natural gas or gasoline. The reforming process is generally performed at a high temperature of 250 to 300° C. In contrast, the system itself may be compact in the direct methanol fuel cell since no reformer is needed, and the power generation process can proceed at a relatively low temperature. Accordingly, the direct methanol fuel cell has been developed in recent years for applying it to a portable power source and an electric car power source by taking notice of this advantage of the direct methanol fuel cell.
An aqueous methanol solution or a vaporized mixture of methanol and water is supplied for feeding methanol and water to the fuel cell in the direct methanol fuel cell generator. Since a vaporizer should be provided as an auxiliary equipment of the fuel cell when methanol and water are supplied as an evaporated mixed gas, the total fuel cell system inevitably becomes large size. On the contrary, the system may be small size when the aqueous methanol solution is supplied, since no vaporizer is needed.
However, there are many difficult problems in the direct methanol fuel cell as described above as compared with solid polymer electrolyte fuel cells.
As an problem, the fuel supplied to the electrode moves within the electrode, enters a proton conductive electrolyte, moves within the electrolyte to arrive at the catalyst, and is used for generating electric power. The proton conductive electrolyte exhibits proton conductivity by being impregnated with water. It has been elucidated in the foregoing studies that introduction of methanol reduces proton conductivity (for example T. J. Chou and A. Tanioka, J. Phys. Chem., B102 (1998), 129). Methanol, water and oxygen as fuel components move by being dissolved into impregnated water in the proton conductive electrolyte in the catalyst layer. Reduced proton conductivity also reduces diffusion of water that moves by being pulled with the protons. Consequently, mobility of water and diffusion of methanol that is completely mixed with water are also decreased at the anode electrode. Methanol also exists at the cathode electrode since methanol supplied to the anode electrode arrives at the cathode electrode through the proton conductive electrolyte membrane. Accordingly, diffusion of water also decreases in the proton conductive electrolyte within the cathode catalyst layer. As a result, diffusion of oxygen is reduced since oxygen diffuses within the electrode by being dissolved in water in the proton conductive electrolyte. In summary, the fuel cell is confronted with a severe problem that diffusion abilities of all the fuel components of methanol, water and oxygen as fuels are reduced. Accordingly, it is inevitable for practical applications to elucidate characteristic values that can be readily measured in close relation with the degree of diffusion of the fuel.
As another problem, diffusion of the fuel is so poor immediately after resumption of operation that equipment is operated under a condition where response to variation of load is very poor, since water impregnated in the proton conductive electrolyte is dried up during pause period of the operation of the direct methanol fuel cell generator. In case of generators, particularly generators for potable appliances and automobiles in which the fuel cell is intermittently operated in daily work and variation of load occurs frequently, the response to variation of load becomes very poor, thereby causing troubles in driving the appliances. Consequently, the trouble may induce severe accidents that may threaten human life. Therefore, it should be confirmed how is the response to variations of load, and how much is the performance of the fuel cell before and during power generation.
As a different problem, the perfluorocarbon sulfonic acid membrane is swelled by being impregnated with water, and swelling is much larger when the membrane is impregnated with methanol. Therefore, the proton conductive electrolyte membrane and catalyst layer are damaged by excessive swelling when a high concentration of aqueous methanol solution is supplied by some reasons, and the performance of the fuel cell is largely decreased.
Consequently, it has been recognized that development of a simple method for deciding the performance of the fuel cell during power generation is also important.
The fuel cell may be severely damaged by the condition of the proton conductive electrolyte membrane, or by the condition of fuel diffusion, in the direct methanol fuel cell generator as described above. Therefore, an inspection method for always verifying the condition of the fuel cell is important.
The characteristics of the fuel cell have been mostly evaluated by measuring an I-V curve. However, the measurements of the I-V curve also involves the results including other lines of information such as catalyst activity and internal resistance other than the degree of diffusion of the fuel. Furthermore, voltages should be measured in a wide range of current density for measuring the I-V curve. In particular, a stationary state operation should be naturally interrupted for measuring the I-V curve of the fuel cell that is under a long-run stationary operation. Since this inspection procedure costs much labor, it cannot be readily applied for evaluation and inspection.